50 vs 85 1.8

Started 2 months ago | Discussions thread
Tom Axford Veteran Member • Posts: 6,064
Re: 50 vs 85 1.8

Dem Bell wrote:

Tom Axford wrote:

Dem Bell wrote:

Say, we can barely make out the letters on a shop far behind the subject when using the 50mm lens. The letters will still be legible if we use the 85 mm lens at the same framing. To blur the letters (so you can no longer read them) one needs a tighter framing, not necessarily a longer lens.

Yes, "tighter framing" is equivalent to focussing on something closer and that puts the background further out of focus.

But "focusing on something closer" will put our subject out of focus. Yes, it will also blur the background more, but it is not the photograph we want to take, right?

When you say tighter framing, you obviously know that that involves both moving closer to the subject and refocussing closer as well.  If you simply left the focus as it was, the subject would be out of focus when you moved closer.  This is so obvious, that I wonder why it needs saying.  Surely it was obvious what I meant.

To spell it out again to avoid any possible misinterpretation (hopefully!):

1. Tighter framing involves moving closer to the subject and refocussing the camera to account for the new subject distance.

2. Simply refocussing the camera to the closer distance (but not moving it) will have exactly the same effect on the background blur, provided the background is at infinity.  Indeed, it is the refocussing that changes the background blur, not moving the camera.

These two things are equivalent in their effect on the background.  They are obviously not equivalent in their effect on the subject!

I hope that explanation makes things clearer.  Changing the focus is what causes the background blur to change.  Moving the camera but leaving the focus distance unchanged will not change the background blur (for a background at infinity).

Correction. In my previous post I missed word "necessarily". One can make framing tighter either by bringing the camera and the subject closer to each other or by changing to a longer focal length.

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